We examined the dropping behaviour of the pea aphid, Acyrthosiphon pisum (Harris), feeding on broad‐bean plants in the laboratory. Aphid responses to foraging and oviposition by females of Monoctonus paulensis (Ashmead) were instar‐specific and included kicking with the hind legs, walking away and dropping from the feeding site. Fourth nymphal instars were most likely to drop, followed by second, third, and first instars, in that order. Compared with first instars, the odds that a fourth‐instar aphid will drop were 6‐times higher independent of the stimulus, and 16‐times higher after parasitoid attack. Dropping from the feeding site increases an aphid's mortality risk. If parasitoid offspring are more likely to survive in small pea aphids, it may be adaptive for M. paulensis to choose smaller hosts, regardless of possible fitness gains due to increased body size from development in larger aphids.

Summary 1.In solitary parasitoid Hymenoptera, normally only one offspring per host survives. Parasitism with more than one egg by the same female results in siblicide and hence is generally maladaptive. 2. The braconid wasp Monoctonus paulensis Ashmead is a solitary parasitoid of aphids, including the Pea Aphid. Although all but one offspring are eliminated by competition between first-instar larvae, females often produce a multiple-egg clutch, even when searching alone. The hypothesis that the size and, presumably, the fitness of the surviving offspring in a clutch are greater than that of singly developing counterparts was tested in the laboratory. Adult size was measured as dry mass in parasitoids developing in Pea Aphids that differed in size (instar) and growth potential. 3. Parasitoid size increased with host size, and females were larger than males. Parasitoids developing in aphids reared under crowded conditions (which had low growth potential) were smaller relative to initial host size. As predicted, the male offspring of virgin mothers developing in a clutch were 4-5% larger than counterparts developing alone. Clutch size had no consistent effect on the size of male or female offspring of mated mothers, however. 4. A female of M. paulensis laying a multiple-egg clutch can gain in fitness in terms of increased offspring size. The surviving offspring has access to greater nutritional resources because superparasitized aphids feed more than single-parasitized aphids; the former also contain proportionately more teratocytes, which are egg-derived cells aiding in parasitoid nutrition. 5. Evidence that some species of solitary parasitoids do better when developing in a clutch suggests that the solitary lifestyle may not be an evolutionary absorbing state.

Abstract. Monoctonus paulensis is a solitary parasitoid of several species of aphids, including the pea aphid, Acyrthosiphon pisum. We evaluated host-instar selection by comparing the parasitoid's preference for the four nymphal instars of the pea aphid, presented two at a time in dichotomous choice tests. Females parasitized more, and laid more eggs in, the relatively smaller aphids among those available. This preference was independent of aphid instar: L1 > L2 > L3 > L4. Preference was not influenced by female size or age. Normal and anaesthetized aphids were accepted equally. The total time needed by a female to capture, position, and parasi tize an aphid varied among host instars, with fourth instars requiring nearly twice as much time as first, second, and third instars. The probability of an attacked aphid escaping or avoiding parasitism increased with aphid instar, from ~10% in first and second instars to ~50% in fourth instars. Although fourth-instar pea aphids contain more resources for offspring development than smaller counter parts, it may not be profitable for a female to invest opportunity time in attacks on large aphids.

Females of Monoctonus paulensis (Ashmead), a solitary parasitoid of aphids, generally select the relatively smaller over equally available larger instars of the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphidoidea: Aphididae). Large hosts contain more resources for parasitoid development and hence have presumably higher quality; however, they require more time to subdue and are more likely to escape. We tested the hypothesis that a female’s choice among first (L1), second (L2), third (L3), and fourth (L4) instars of the pea aphid is based on the optimal balance between fitness costs in terms of time and fitness gains in terms of offspring number and size. Prepupal mortality did not vary with host instar, but pupal mortality was greater among parasitoids developing in L4 than in any younger instars. Offspring mortality was not influenced by clutch size in that mortality risk did not differ between parasitoids developing alone and counterparts developing in a clutch. The sex ratio, measured as proportion of daughters among offspring, was female-biased on all four host instars; the degree of bias increased from 0.70 (in L1) to 0.92 (in L4). Parasitoid body size was a function of aphid size at parasitism. Females were larger than males; the magnitude of the difference in body size was constant and independent of host and hence parasitoid size. A female’s potential fecundity as measured by the number of ovarial eggs at eclosion varied with her size and larval ontogeny. The four instars of the pea aphid were ranked in the order L1 > L2 > L3 > L4 both in terms of the number of offspring produced per encountered host and in terms of a female’s time costs; first instars are easier to handle and are more abundant in the field than older instars. The four host types were ranked in the order L2 > L3 > L1 > L4 in terms of the proportion and potential fecundity of daughters among offspring. The observed preference pattern (L1 > L2 > L3 > L4) suggests that, in choosing hosts, females of M. paulensis maximize the number of offspring per unit of search time rather than simply offspring quality.

No abstract